High Energy Astrophysical Phenomena (astro-ph.HE)

Abstract: Magnetars are a unique class of neutron stars characterized by their incredibly strong magnetic fields. Unlike normal pulsars whose X-ray emission was driven by rotational energy loss, magnetars exhibit distinct X-ray emissions thought to be driven by their strong magnetic fields. Here we present the results of magnetar X-ray spectra analysis in their quiescent state. Most of the spectra of magnetars can be fitted with a model consisting of a power-law and a black body component. We found that the luminosity of the power-law component can be described by a function of black body temperature and its emission radius. The same relation was seen in pulsars whose X-ray emission mechanism is thought to be different. The fact that magnetars and pulsars share a common fundamental plane in the space spanned by non-thermal X-ray luminosity, surface temperature, and the radius of the thermally emitting region indicates that further fundamental information is necessary to gain a complete comprehension of the magnetospheric emissions from these two classes of neutron stars.

Abstract: We report the discovery of a 2.11 ms binary millisecond pulsar during a targeted search of the redback optical candidate coincident with the $\gamma$-ray source 3FGL J0212.5+5320 using the Robert C. Byrd Green Bank Telescope (GBT) with the Breakthrough Listen backend at L-band. Over a seven month period, five pointings were made near inferior conjunction of the pulsar in its 20.9 hr orbit, resulting in two detections, lasting 12 and 42 minutes. The pulsar dispersion measure (DM) of 25.7 pc cm$^{-3}$ corresponds to a distance of 1.15 kpc in the NE2001 Galactic electron density model, consistent with the Gaia parallax distance of $1.16\pm0.03$ kpc for the companion star. We suspect the pulsar experiences wide-orbit eclipses, similar to other redbacks, as well as scintillation and DM delays caused by its interaction with its companion and surroundings. Although the pulsar was only detected over $\approx3.7\%$ of the orbit, its measured acceleration is consistent with published binary parameters from optical radial velocity spectroscopy and light-curve modeling of the companion star, and it provides a more precise mass ratio and a projected semi-major axis for the pulsar orbit. We also obtained a refined optical photometric orbit ephemeris, and observed variability of the tidally distorted companion over 7 years. A hard X-ray light curve from NuSTAR shows expected orbit-modulated emission from the intrabinary shock. The pulsar parameters and photometric ephemeris greatly restrict the parameter space required to search for a coherent timing solution including pulsar spin-down rate, either using Fermi $\gamma$-rays, or further radio pulse detections.

Abstract: Magnetic fields in galaxies and galaxy clusters are believed to be the result of the amplification of intergalactic seed fields during the formation of large-scale structures in the universe. However, the origin, strength, and morphology of this intergalactic magnetic field (IGMF) remain unknown. Lower limits on (or indirect detection of) the IGMF can be obtained from observations of high-energy gamma rays from distant blazars. Gamma rays interact with the extragalactic background light to produce electron-positron pairs, which can subsequently initiate electromagnetic cascades. The $\gamma$-ray signature of the cascade depends on the IGMF since it deflects the pairs. Here we report on a new search for this cascade emission using a combined data set from the Fermi Large Area Telescope and the High Energy Stereoscopic System. Using state-of-the-art Monte Carlo predictions for the cascade signal, our results place a lower limit on the IGMF of $B > 7.1\times10^{-16}$ G for a coherence length of 1 Mpc even when blazar duty cycles as short as 10 yr are assumed. This improves on previous lower limits by a factor of 2. For longer duty cycles of $10^4$ ($10^7$) yr, IGMF strengths below $1.8\times10^{-14}$ G ($3.9\times10^{-14}$ G) are excluded, which rules out specific models for IGMF generation in the early universe.

Abstract: The extragalactic magnetic field (EGMF) could be probed with $\gamma$-ray observations of distant sources. Primary very high energy (VHE) $\gamma$-rays from these sources absorb on extragalactic background light photons, and secondary electrons/positrons from the pair production acts create cascade $\gamma$-rays. These cascade $\gamma$-rays could be detected with space $\gamma$-ray telescopes such as Fermi-LAT. The $\gamma$-ray burst GRB 221009A was an exceptionally bright transient well suited for intergalactic $\gamma$-ray propagation studies. Using publicly-available Fermi-LAT data, we obtain upper limits on the spectrum of delayed emission from GRB 221009A during the time window of 30 days after the burst, and compare these with model spectra calculated for various EGMF strengths $B$, obtaining lower limits on $B$. We show that the values of $B < 10^{-18}$ G are excluded. For some optimistic models of the VHE spectrum of GRB 221009A, the values of $B < 10^{-17}$ G are excluded.